Secondary recovery process using gasliquid drive fluids



United States Patent Ofitice 3,1?7,939 Patented Apr. 13, 1055 3,17'7fi39SEQGNDARY PTECQVERY PROCESS USH'JG GAS- DRWE FLUIBS Le Eloy VI. Helm,Crystal Lake, ill and Albert if. (Isaszar, @lzlahorna (Iity, @kia,assignors to The Pure Gil Company, Palatine, IlL, a corporation of OhioN Drawing. Filed Jan. 31, 1963, Ser. No. 255,161 to tllaiins. (Cl. 1669)This invention relates to the recovery of petroleum from undergroundreservoirs, and is more particularly concerned with a flooding processfor recovering petroleum in greater amounts than is possible byconventional floodring techniques.

It is Well known in the secondary recovery art to inject a slug ofsolvent which is miscible with both the petroleum oil in the reservoirand with water, and to drive this slug of fluid through the reservoir byinjecting floodwater behind it. Typical of the solvents used in suchsecondary recovery processes are lower alcohols. It has also beenproposed to inject a slug of hydrocarbon solvent, usually of lowmolecular weight, and then inject a slug of solvent which is misciblewith both the petroleum oil and with water. Finally, fioodwater isinjected. The presence of large volumes of Water in the reservoirhinders the removal of petroleum by such miscible phase displacementprocesses. lVhere alcohol alone is used as the solvent, .the alcoholdissolves in the reservoir Water, deteriorating the alcohol bank.Because of the high solubility of the injected solvent in the reservoirwater, the leading edge of the solvent zone becomes diluted with water,and the solvent zone may break down entirely, with the result that theoil-to-solvent-to-water miscible displacement is lost.

When a first slug of hydrocarbon solvent is used prior to the alcohol,the presence of large volumes of water in the reservoir still stronglyhinders oil recovery, although the reason is obscure, since the firstinjected hydrocarbon would be exepected to drive formation water aheadof it and prevent contamination of the alcohol bank.

It has now been found that an improvement of the dual slug type miscibledisplacement process can be obtained in treating reservoirs containinghigh concentrations of reservoir water by first injecting a slug of agas into the reservoir, before the injection of the hydrocarbon solvent.It has been found that still greater improvement in recovery ofpetroleum from reservoirs having high waterto-oil ratios can be obtainedif a foam is injected through the input well and into the formation orproduced downhole, prior to the injection of solvent.

Briefly, this invention teaches a process for the recovcry of oil fromreservoirs containing high concentrations of reservoir Water comprisingthe steps of first injecting a small slug of an aqueous solution of highfoaming surfactant into the formation, then injecting a slug of gaswhich is substantially insoluble in the petroleum contained in reservoirand has only limited solubility in water, then injecting a quantity ofhydrocarbon solvent, followed by a solvent such as an alcohol which issoluble in both petroleum and fioodwater, and finally injectingfloodwater to drive the "as and solvent through the formation to aproducing Well. Alternatively, the injection of aqueous surfactantsolution can be omitted. The function of the aqueous surfactant solutionand gas injected prior to the solvent is to produce a stable foam in theformation, which foam is effective in reducing the water saturationahead of the solvent bank and thereby preventing deterioration of thesolvent bank.

it is an object of this invention to provide an improved method for thesecondary recovery of oil from underground reservoirs.

Another object of tihs invention is to provide a method for recoveringhigher percentages of petroleum in reservoirs which contain highconcentrations of water.

The injection of aqueous surfactant solution and gas having lowsolubility in petroleum, prior to the injection of liquid hydrocarbonsolvent and a second solvent which is soluble in both the petroleum oiland the subsequently injected fioodwater, is contrary to principlestaught in the prior art. The prior art considers the injection of ahighly mobile gas before the injection of a miscible solventdisadvantageous in that it results in disruption of continuity in theoil phase in the reservoir, and tends to cause fingering and lay-passingof oil-containing zones, and thereby reduces flood emciency. It hasnevertheless been found that where the reservoir to be treated containswater in amount of about 0.5 pore volume or more, the disrupting efiectsof the .gas injected prior to the miscible solvent is minimized, and theefficiency of the process is enhanced. Accordingly, it has been foundthat by injecting a small amount of aqueous surfactant solution and alarger amount of gas prior to the injection of miscible solvents, whentreating reservoirs having a water concentration of not less than about0.5 pore volume, the efficiency of recovery is raised far above thatpossible by conventional solvent flooding.

In carrying out the process of this invention, a small slug of waterhaving a volume in the range of about 0.005 to 0.05 reservoir porevolume, preferably about 0.02 reservoir pore volume, containing a smallamount of surfactant of the high-foaming type, sufiicient in amount toproduce a stable foam when the Water slug is contacted by a gas, isinjected. The concentration of surfactant is ordinarily in the range of0.1 to 5.0% by weight of the water slug. Following the injection of theaqueous surfactant solution, a slug of gas having low solubility inpetroleum and a volume of about 0.1 to 0.5 reservoir pore volume isinjected through the input Well and into the pctrcleum reservoir. Thevolume of gas injected must be sufficient to form a foam bank which willsubstantially displace the formation water and maintain an effectivebuffer between the formation water and the later-injected solvent. Ingeneral, the lowest effective amounts of gas will be most suitable,since the injection of a gas slug larger than is required could have anadverse efiect upon the sweep efficiency of the process, therebypreventing maximum oil recovery. While quantities of gas in the range of0.1 to 0.5 pore volume may be employed, in most instances it will bepreferred to use not more than about 0.2 pore volume of gas measured atinjection temperature and pressure. Next, a slug of liquid hydrocarbonsolvent is injected. The amount of hydrocarbon will be in the range of0.03 to 0.20 reservoir pore volume, and preferably about 0.10 porevolume.

A slug of a suitable solvent which is soluble in both petroleum oil andwater, and preferably is miscible with both oil and water, is injectedinto the reservoir behind the hydrocarbon sing. The volume of thatsolvent injected is not critical, but quantities less than 0.03 porevolume are seldom effective, and quantities in excess of about 0.2reservoir pore volume increase the expense of the process withoutproducing a proportionate increase in the oil ultimately recovered.Finally, floodwater is injected in a conventional manner to drive thepreviously injected gas and solvent slugs towards the producing well.This final water injection is continued until the Water-to-oil ratio atthe producing well rises to such a level that further oil recoverybecomes uneconomical, at which time injection is terminated. Thisprocess is applicable to any of the standard flooding patterns, such asthe line drive, five-spot, or nine-spot systems.

A Wide variety of gases are available which may be used in carrying outthe process of this invention. The gas chosen must have a solubility inpetroleum not greater than about 10% by weight at reservoir temperatureand pressure, and should be substantially inert, that is, chemicallynon-reactive under reservoir conditions. It is also desirable, but notessential, that the gas have a solubility not greater than about 10% byweight in the water. Suitable gases include, but are not limited tonitrogen, flue gas, air and natural gas which is substantially free ofcomponents heavier than ethane.

The injected solvent can consist of a hydrocarbon fraction of the C to Crange. Exemplary materials are LPG, propane and naphtha. Especiallypreferred are materials such as catalytic reformate, which contain morethan 50 percent aromatics and/ or olefins.

Similarly, a wide variety of materials are available which may be usedas the second injected solvent, it being required only that the solventbe liquid in physical state, capable of dissolving at least 10% of itsweight of petroleum, and soluble in water to the extent of at least 10%of the weight of the water. Preferably, the solvent will be misciblewith both petroleum oil and water, and still more preferably will have apartition coefficient favoring solution in oil when in the presence ofboth oil and water. Typical suitable solvents are partially oxidizedhydrocarbons such as alcohols, including tertiary butanol, secondarybutanol, n-butanol, isopropanol, n-propanol, and pentanols. Othersuitable materials include ketones, such as methylethyl ketone, diethylketone, and di-npropyl ketone; aldehydes such as acetaldehyde,propionaldehyde and butyraldehyde, and organic acids such as aceticacid, propionic acid, butyric acid, and isobutyric acid. Mixtures of theaforenamed liquids may also be used in the process of this invention.Mixtures of partially oxidized hydrocarbons produced by hydrocarbonoxidation processes comprising a wide variety of alcohols, aldehydes,ketones and acids, and other constituents, in mixture, may be usedprovided the weighted average of the solubilities of the constituents ofthe mixture in petroleum is suitably high. Such mixtures may containminor amounts of material having a low solubility in petroleum, such asmethyl and ethyl alcohols, provided the quantities of such constituentsare not excessive and other more soluble constituents are present inquantity. Mixtures of C and C alcohols, ketones, aldehydes, and organicacids are suitable.

The foaming surfactant employed in the aqueous surfactant slug must bewater-soluble at least to the extent in which it is incorporated.Suitable water-soluble surfactants include iso-octyl phenyl polyethoxyethanols, having about 9 to 10 ethoxy groups per molecule, sulfonatedalkyl esters, and sodium lauryl ether sulfate. These materials aremerely exemplary of a wide variety of high-foaming, water solublesurfactants which are reasonably compatible with reservoir water andcapable of producing large quantities of stable foam when contacted by agas. -Other suitable surfactants can be selected from published listsdescribing numerous surfactants and their properties.

The term high-foaming surfactant as used in this specification and theappended claims denotes a surfactant which, when placed in solution inwater in the amount of by weight or less, is capable of producing largequantities of stable foam. The suitability of surfactants for use in themethod of this invention may be experimentally determined byincorporating 5% or less, by weight, of the selected surfactant inwater, which is preferably typical in salt content to the water to beinjected. About 500 milliliters of the aqueous surfactant solution isplaced in a graduated cylinder to form a liquid column 50 centimeters inheight. Gas is then introduced from the bottom of the column through afritted glass disc, at substantially atmospheric pressure; and permittedto bubble upward through the column, at a rate of 500 milliliters of gasper minute, per square centimeter of column crosssectional area. Afterthe gas has bubbled through the column of liquid for a period of about15 minutes, the height of the column of foam produced above the liquidis measured, and should be not less than about 150 centimeters.High-foaming surfactants, in accordance with this specification, arethose which will meet the aforedefined test. It is preferred to employsurfactants capable of meeting the aforedescribed test with respect tofoam quantity when the amount of surfactant employed is substantiallyless than 5% by weight of water. In general,

it is preferred to maintain the amount of surfactant added.

at a minimum, both for reasons of economics, and to avoid the productionin the formation of a foam of excessive thickness which may tend to plugthe formation. It will be understood that the quantity of surfactant tobe added may vary depending upon the salt content of the water employed,but that the weight of surfactant will be in the range of 0.5 to 5% andthat the best amount may be readily determined by the aforedescribedexperiment.

The effectiveness of the method of this invention has been establishedby comparative experiments. The experiments were conducted in 3 /2 inchdiameter, 8 foot long Berea sandstone cores using water-driven solvents,

at F. and 1400 p.s.i.g. The tabulation below reports the experimentalresults.

1 Measured at 1400 p.s.i.g. and room temperature.

Experiment 4 establishes the effectiveness of the method of thisinvention over similar processes in which the injection of hydrocarbon,or gas, or both, are omitted. In

Experiment 4, although no aqueous surfactant was injected, all of theoil in place and 15.5% of the catalytic reformate (hydrocarbon) injectedwas recovered.

In long systems, as exist in actual field operations, an increase. inthe amount of oil and/or hydrocarbon solvent recovered is obtained byinjecting a slug of aqueous surfactant solution before the injection ofgas.

As a specific example of the method of this invention a petroleumreservoir penetrated by wells in five-spot pattern, which has beensubjected to waterfiood and now contains 0.60 pore volume of water, 0.30of oil, and 0.10 of gas, is treated as follows: Through the central wellof the pattern are injected in sequence 0.15 pore volume of air, 0.05pore volume of liquid propane, 0.05 pore volume of isopropyl alcohol,and floodwater. Petroleum oil is recovered from the other four wells ofthe pattern. The foregoing procedure may be modified to the extent ofshutting in the production wells while the air is being injected intothe injection well of the well pattern.

As another example of'the method of this invention, the foregoingexample is modified to the extent of first injecting 0.03 reservoir porevolume of an aqueous solution containing 3% by weight of sodium laurylether sulfate. tained in the foregoing example are realized.

The embodiments of the invention in which an exthrough an input well andinto said reservoir about 0.1' to 0.5 pore volume of a gas having asolubility in petro- Oil recoveries slightly higher than those obleumnot greater than about 2% by weight, then injecting 0.03 to 0.20 porevolume of liquid C to C hydrocarbon, thereafter injecting about 0.03 to0.20 pore volume of a liquid solvent capable of dissolving at least 10%its weight of petroleum and soluble in water to at least 10% of theWeight of the water, driving the injected material through saidreservoir by the injection of fioodwater producing petroleum from anoutput well.

2. The method in accordance with claim 1 in which the principalconstituent of said gas is nitrogen.

3. The method in accordance with claim 2 in which said gas is air.

4. The method in accordance with claim 3 in which said producing Well isshut in While air is injected through the input Well.

5. The method in accordance with claim 3 in which said liquid solventconsists essentially of oxygenated hydrocarbons having 3 to 7 carbonatoms per molecule.

6. The method in accordance With claim 5 in which a major proportion ofthe (3 43 hydrocarbons are selected from the group consisting of olefinsand aromatics.

7. The method in accordance with claim 6 in which the volume of solventinjected is about 0.1 pore volume.

8. The method in accordance with claim 7 in which the volume ofhydrocarbons injected is about 0.1 reservoir pore volume.

9. The method in accordance with claim 8 in which the volume of airinjected is about 0.1 to 0.5 reservoir pore volume.

10. The method in accordance with claim 1 including 5 the step ofinjecting through said input Well about 0.005 to 0.05 reservoir porevolume of an aqueous solution of a high foaming surfactant, saidsurfactant being present in sufiicient quantity to produce a stable foamwhen said solution is contacted by gas, said solution being injectedprior to the injection of said gas.

11. The method in accordance With claim 10 in which the principalconstituent of said gas is nitrogen.

12. The method in accordance with claim 11 in which said gas is air.

13. The method in accordance with claim 12 in which said liquid solventconsists essentially of oxygenated hydrocarbons having 3 to 7 carbonatoms per molecule.

14. The method in accordance with claim 13 in which a major proportionof the C -C hydrocarbons are selected from the group consisting ofolelins and aromatics.

15. The method in accordance with claim 14 in which the volume of airinjected is about 0.1 to 0.5 reservoir pore volume.

16. The method in accordance with claim 15 in which said solutioncontains 0.1 to 5.0 percent by weight of surfactant.

References Cited by the Examiner UNITED STATES PATENTS 2,742,089 4/56Morse 1669 2,866,507 12/58 Bond 166-9 3,080,917 3/63 Walker i 166-9CHARLES E. OCONNELL, Primary Examiner. BENJAMIN HERSH, Examiner.

1. THE METHOD FOR THE RECOVERY OF PETROLEUM OIL FROM RESERVOIRSCONTAINING WATER IN EXCESS OF ABOUT 0.5 PORE VOLUME AND PETROLEUMCOMPRISING FIRST INJECTING THROUGH AN INPUT WELL AND INTO SAID RESERVOIRABOUT 0.1 TO 0.5 PORE VOLUME OF A GAS HAVING A SOLUBILITY IN PETROLEUMNOT GREATER THAN ABOUT 2% BY WEIGHT, THEN INJECTING 0.03 TO 0.20 POREVOLUME OF LIQUID C3 TO C10 HYDROCARBON, THEREAFTER INJECTING ABOUT 0.03TO 0.20 PORE VOLUME OF A LIQUID SOLVENT CAPABLE OF DISSOLVING AT LEAST10% ITS WEIGHT OF PETROLEUM AND SOLUBLE IN WATER TO AT LEAST 10% OF THEWEIGHT OF THE WATER, DRIVING THE INJECTED MATERIAL THROUGH SAIDRESERVOIR BY THE INJECTION OF FLOODWATER AND PRODUCING PETROLEUM FROM ANOUTPUT WELL.